Should you own any of the following hats in TF2, you'll now be able to wear them in Portal 2 too:

Mann Co. Cap

Prince Tavish's Crown

Pyro's Beanie

Fancy Fedora

Master's Yellow Belt

Tyrant's Helm

Portal 2, if you were unaware, has character customisation for the two co-op bots. Players can equip Atlas and P-Body with new gestures, paint patterns, accessories, and, of course, hats. Some can be unlocked with achievements, while many more are sold through the Robot Enrichment program--essentially a Portal 2 version of TF2's Mann Co. Store.

Angry, reactive and foolish corners of the Internet, this is your cue to proclaim that hats have ruined Portal 2.